slight concern on my plants during flushing

hello everyone.im a little concerned with my plants.i am in the very final stages of there lives,i have been flushing them with 2 litres of water every 3 days now for nearly 3 weeks and i have just noticed some of the leaves have got yellow tips emerging.i have been growing in soil using hesi soil fertilisers at 1/2 to 3/4 strength and the variety is some sort of kush and they are under a 600w hps light with good air movement in and out.i have had absoloutely no problems this time around so im wondering why these tips are appearing now right at the end and especially after 18 days of flushing.any ieas as to why or am i worrying for no reason now im so close to the end? i was looking at cutting down probably in the next week anyway. thanks